There has been considerable interest in recent years in developing high energy density batteries with lithium containing anodes. Typical rechargeable batteries of this type include an anode with lithium metal as the active anode species or a lithium intercalation compound as the active anode species. Many of these batteries include a cathode including sulfur as the active cathode species.
Upon charging rechargeable batteries of this type, lithium ion is reduced to lithium metal at the anode while, at the cathode, lithium sulfide species are oxidized to form sulfur. Sulfur generated in this process is incorporated into other sulfur defining a portion of the cathode. Lithium ion is released into an electrolyte connecting the cathode with the anode. Upon discharge, lithium metal at the anode is oxidized to lithium ion, which is released into the electrolyte, while at the cathode lithium ion and sulfur engage in a reduction reaction to form a lithium sulfide species.
Batteries of this type are particularly attractive in terms of weight and energy density, especially those including lithium metal as an active anode species. Lithium metal anodes, or those comprising mainly lithium metal, provide an opportunity to construct cells that are lighter in weight, and which have a higher energy density than cells such as lithium-ion, nickel metal hydride or nickel-cadmium cells. These features are highly desirable for batteries for portable electronic devices such as cellular phones and laptop computers where a premium is paid for low weight.
As noted above, lithium polysulfide species (also referred to herein as “polysulfides”) play a role in the chemistry of such batteries. Upon discharge, lithium polysulfide species are formed during a reduction reaction at the sulfur cathode, involving lithium ion from the electrolyte. As is also known, a shuttle mechanism can exist in batteries of this type involving oxidation and reduction of lithium sulfides from higher-order species containing more sulfur to lower-order species containing less sulfur, for example, Li2S.
In some rechargeable lithium batteries of this type, the use of a single electrolyte is not optimal for both the anode and cathode, e.g., high sulfur cathode performance may be achieved, but at the expense of lithium anode cycle-ability and stability. For instance, to obtain better sulfur cathode performance, rate capability, and sulfur utilization, a suitable electrolyte that can dissolve polysulfides at high concentrations may be chosen. However, such an electrolyte, in addition to the dissolved polysulfides in the electrolyte, may cause lithium anode corrosion. On the other hand, when an electrolyte that is less reactive towards the lithium anode is used, polysulfide solubility in such an electrolyte may be relatively poor. This can cause buildup of insoluble polysulfides (i.e., “slate”) at the cathode, which can result in poorer device performance and/or shorter device life. Accordingly, methods and devices involving electrolytes that are favorable towards both the anode and cathode would be desirable.